This is an odd one for sure.
It almost sounds like the variator is acting too quickly, not too slowly. From my understanding, two things control the state of the variator - RPM and load.
If the variator was acting too quickly, it would hobble the engine with too high of a gear ratio. The engine might not be able to get into the powerband. Simply not making enough horsepower to overcome the ever-increasing gear ratio.
When under load (on the uphill) the load is enough to keep the variator at a lower ratio for the same RPM - so now the engine can get further into the powerband, and make a little more speed.
Does the PA50 variator have return springs for the weights? Possible that the spring(s) are sacked? Otherwise, it seems that the engine is just not making enough power at the RPM the variator system is designed for.